The accurate and continuous control of the thickness of a coating being applied to the surface of a sheet material is required in a number of industries. For example, it is often necessary to control the thickness of coatings applied to paper or other similar materials in the form of a continuous travelling web. Another field in which the control of thickness of a continuously applied coating is required is in hot dip galvanising of metal sheet. The present invention has been particularly developed for controlling the coating thickness in a hot dip galvanising operation and it will be convenient to particularly describe the invention in relation to this field of application. However it should be appreciated that the apparatus and process of the present invention is applicable to other coatings and to other sheet materials being coated.
The hot dip galvanising process aims to apply a controlled amount of a protective coating, typically zinc and/or zinc alloy, to the surface of a metal item in order to produce a reasonable corrosion service life for the coated component. In the case of sheet metal for use in automobiles, roofing and similar applications a continuous hot dip galvanising line is employed to coat the sheet. The basic elements of such a galvanising line are depicted in FIG. 1. The galvanising line 1 includes input coils 2 which carry a roll of metal sheet 3. The metal sheet 3 is fed into an accumulator 4 and then passed through a furnace 5 (not shown). Upon leaving the furnace the metal sheet is passed through a bath of protective coating 6, such as molten zinc. The coated sheet 11 then passes between a pair of air knives 15 located on opposing sides of the sheet. Scanning X-ray gauges 47 are located down the line beyond the air knives 15 to monitor the resulting thickness of the protective coating.
For metal sheet processed upon such a line the corrosion service life of the final product is dictated by the thickness of the applied coating, and in particular by the thinnest coating at any point of the applied coating. This is reflected in the standard measures which are used to classify coated sheet, which focus upon the minimum single spot thickness in the coating. Due to such standards specifying a minimum coat requirement, the producers of galvanised sheet must ensure that at all points over the sheet the coat thickness is above the minimum required for the product in question.
The coating thickness is controlled by the application of a high pressure wiping jet which issues from an air knife adjacent to each surface of the metal sheet as the sheet is withdrawn from a molten zinc bath. A typical air knife 15 is depicted schematically in FIG. 2. The amount of coating which remains upon the surface of the sheet after passing through the wiping jet is mainly determined by the following parameters:
line speed, PA1 distance from the air knife mouth to the sheet surface, PA1 pressure within the air knife header, and PA1 dimensions of the mouth. PA1 a body for mounting adjacent to a sheet material which in use is moved past the body in a longitudinal direction; PA1 a mouth facing the sheet material and extending laterally, the mouth being defined by a pair of facing lips, the facing lips movable relative to the body; PA1 means for distributing a fluid under pressure through the body to enter and emerge from the mouth whereby pressurised fluid emerging from the mouth operates on a coating applied to a surface of the sheet material passing the air knife to thereby control the thickness of the coating; PA1 moving means operable to move the lips relative to the air knife body; PA1 wherein the pair of facing lips are laterally displaceable relative to each other and are configured and adapted to co-operate with each other such that upon relative lateral displacement of the lips the separation between the lips is varied along at least part of the length of the mouth; PA1 the moving means being operable to laterally displace the lips relative to each other so as to vary the separation of the lips and hence the width of the mouth at different points along at least part of the length of the mouth. PA1 wherein the pair of facing lips are laterally displaceable relative to each other and are configured and adapted to co-operate with each other such that upon relative lateral displacement of the lips the separation between the lips is varied along at least part of the length of the mouth.
The distance from the air knife mouth to the sheet surface is controlled by horizontal positioning of the air knife assembly and is dependent also on the profile which the sheet adopts as it passes the air knives. Referring to the horizontal section shown in FIG. 3a, a bowed sheet profile can occur as the sheet exits the molten zinc bath and passes the air knives. This reduces the knife mouth to sheet distance in the centre section of the upper or distal sheet surface 31 relative to the knife mouth to sheet distance in the edge regions of sheet surface 31. The reverse is true for the lower or proximal sheet surface 32. This has the effect of introducing a variation in the thickness of the coating across the sheet. FIG. 3b illustrates a typical coating profile on the upper or distal sheet surface 31 whilst FIG. 3c illustrates a typical coating profile on the lower or proximal sheet surface 32. This coating defect is known as "crossbow".
The crossbow defect in the coating profile forces line operators to increase the average amount of coating material which is applied to the sheet surfaces in order to ensure that the minimum single spot requirements are satisfied across the entire width of the sheet. Typical magnitudes of crossbow defect are of the order of 10% of the average coating being applied. As such, the over-coating required in order to avoid crossbow induced violations of minimum single spot coating requirements represents a significant cost to continuous galvanising line operators.
There have been many systems proposed or implemented for controlling the coating thickness, particularly in circumstances where bowing of the sheet material causes variations in coating thickness across the width of the sheet. Such systems have typically involved modifying the configuration or other parameters of the air knife. For example, there have been proposed systems for changing the shape of the mouth of the air knife through which the pressurised gas emerges by deforming one of the lips defining the mouth; the deformation of the lip being non-uniform along its length so that the width of the mouth opening varies at different points along its length. Constricting the width of the mouth reduces the gas flow at that point thus reducing the wiping effect at that point and thereby producing an increased coating thickness, and vice versa. Examples of such systems involving selective opening and closing of the width of the mouth are shown in the published patent specifications U.S. Pat. No. 5,423,913 and AU-37005/93.
In patent specification AU-50750/85 there is proposed a system of modifying the gas flow through the mouth of the air knife by selectively opening fluid escape passages which open into the mouth to thereby locally reduce the flow rate through the mouth.
U.S. Pat. No. 4,524,716 discloses an air knife in which movable obstructions are provided inside the mouth so as to selectively modify the fluid flow through the mouth and thereby provide different flow rates at different points across the sheet or web being coated.
All of these prior proposed systems are mechanically relatively complex since they require a multiplicity of controllable operating components provided along the length of the mouth. This can make the total air knife assembly and control system complex in construction and operation and susceptible to mechanical failure or dysfunction, and difficult expensive or time consuming to install, calibrate or repair.
Accordingly, it is an object of the present invention to provide an air knife arrangement for use in coating thickness control which overcomes or ameliorates at least one shortcoming of prior air knife arrangements, or at least provides a useful alternative to prior air knife arrangements.
Although the terms "air knife", "air knife assembly", and "air knife arrangement" are used throughout the specification and claims such terms are not to be construed as limiting the invention to a field of application in which pressurised air is used. Other gases, or even liquids, may be used and are encompassed by the invention.